Allow SetLevel to set a level that mutes all logs (not just up to Fatal)

[tonglil]

Is your feature request related to a problem? Please describe.
I want to be able to call SetLevel and turn off all logging for the logger.

Describe the solution you'd like
SetLevel(level.Off) or documented method of doing this.

Describe alternatives you've considered
I've looked through godocs and issues and cannot find a way to do this with the keywords: off, noop, disable, all

Additional context
n/a

[aymanbagabas]

You could use Level(math.MaxInt)